Of all the changes that occur in a woman's body during pregnancy, the most physically apparent is the abdominal distension that accompanies a growing baby. Pregnant women are all different, and while some may look relatively small throughout their pregnancies, others appear to have huge bellies. Particularly when a woman's belly appears large early--near the beginning of the third trimester, for instance--it's common to wonder what's causing the distension.
A huge belly at 29 weeks pregnant may be meaningful, or may mean nothing more than that a particular woman's body habitus lends itself to a larger-than-normal-appearing abdomen. In his book "Anatomy and Physiology," Dr. Gary Thibodeau notes that during pregnancy, a woman's uterus grows so large that it forces her other abdominal organs upward, toward the thorax. The more room she has through the upper body--in other words, the longer-waisted she is--the less likely her abdomen is to protrude. Instead, there is room for the baby, and everything else, along the length of the torso. Shorter-waisted women tend to protrude more.
While almost every woman's body changes significantly at some point in pregnancy with regard to waistline, some women protrude much earlier than others. Heidi Murkoff and Sharon Mazel explain in their book, "What To Expect When You're Expecting," that bellies "pop" at different points in pregnancy. Further, they note that second-time mothers tend to expand more quickly and to a greater extent than first-time mothers, meaning that a huge belly at 29 weeks is more likely in later pregnancies.
Other factors may also play a role in the extent to which a belly protrudes. In his book "What You Didn't Think To Ask Your Obstetrician," Dr. Raymond Poliakin suggests that very toned women with strong abdominal muscles tend to hold their babies in tighter until very late in pregnancy. This contributes to fit women showing later, and to a lesser extent, than their less toned counterparts. Women who rarely engage the abdominal muscles are likely to show large bellies much earlier, and may look nearly full term at 29 weeks.
There's another possible explanation for a truly huge belly at 29 weeks, but these days, it's rare. Undiagnosed twins--that is, twins that neither the obstetrician nor the mother expects--were once relatively routine, note Doctors Shawn Tassone and Kathryn Landherr in their book, "Hands Off My Belly." With ultrasound as a standard of care in modern obstetrics, it's extremely unlikely that a woman could be pregnant with twins and not know it. Still, there is a small possibility, particularly if a woman has only a single ultrasound or refuses all ultrasounds during her pregnancy, that she could be pregnant with twins. A particularly large belly at 29 weeks carries with it the potential for multiple babies.
Women who are heavy at the beginning of their pregnancies tend to look smaller through the belly relative to those who are quite slender, note Murkoff and Mazel. This is simply a function of relative size--on a very slim woman, a distended belly looks huge. Further, obstetricians generally advise heavier women to gain less weight during pregnancy, since this results in healthier babies. As a result, heavy women may actually lose fat, though they'll still gain weight, during their pregnancies, resulting in a smaller-appearing belly. Slim women are advised to gain 25 to 35 lbs., which can exacerbate the effects of belly protrusion, and could explain a huge belly at 29 weeks.